Capacitive transducers include strain gages, pressure transducers and accelerometers. Typically, such transducers are utilized in a half-bridge configuration wherein the parameter being measured causes the capacitance of one or both capacitors of a capacitor pair to vary.
Capacitive strain gages are known in the prior art for measuring strain in a test specimen when the specimen is subjected to a mechanical force. Typically, capacitive strain gages have utilized capacitors with plates which are movable relative to each other as a function of applied strain. As a force is applied to the test specimen, relative movement of the capacitor plates causes the capacitance to change. The change in capacitance is sensed by detecting the change in an applied a.c. signal.
In a conventional prior art configuration, a capacitive strain gage utilizes two capacitors in a half-bridge configuration. Sine wave voltages of equal amplitude and opposite phase are applied to the two capacitors. In a quiescent condition, the two capacitors are equal, and the output of the strain gage is zero, since the out-of-phase excitation signals cancel. When a strain is applied to the test specimen, one of the capacitors changes value and the half-bridge is unbalanced, resulting in an output signal representative of the applied strain. A signal conditioner is used to generate the excitation signals and to amplify and demodulate the output signal from the strain gage.
In order for a capacitive strain gage or other capacitive transducer to provide an accurate measurement, the sinusoidal excitation waveform must be stable, both in amplitude and frequency. Conventional oscillators do not have the required amplitude and frequency stability. The signal conditioner must also provide linear demodulation of the output signal from the transducer. A signal conditioner for a capacitive strain gage utilizing a four quadrant multiplier notch filter demodulator is disclosed in U.S. Pat. No. 3,852,672 issued Dec. 3, 1974 to Nelson.
It is a general object of the present invention to provide an improved signal conditioner for a capacitive transducer.
It is another object of the present invention to provide a highly accurate signal conditioner for use with a capacitive transducer.
It is a further object of the present invention to provide a circuit for generating a sinusoidal voltage, that is stable in both amplitude and frequency.
It is yet another object of the present invention to provide a signal conditioner that is low in cost and easy to manufacture.
It is a further object of the present invention to provide a highly linear signal conditioner for use with a capacitive transducer.